Hydraulic dampers or gas springs used in automobiles or two-wheeled vehicles are either a single cylinder type comprising one cylinder or a multi-cylinder type comprising an outer cylinder and inner cylinder. In both cases, the inside of the cylinder is divided into two chambers by a piston. A piston rod has an end connected to the piston. The piston rod is supported and guided free to slide by a rod guide in the cylinder, this rod guide acting as a bearing unit at the open end of the cylinder.
A rod guide which functions as a piston rod bearing in this way is disclosed in Tokko Sho 62-38574, Jikko Sho 55-23226 and Jikkai Hei 5-47569 published by the Japanese Patent Office.
The rod guide described in Tokko Sho 62-38574 is press-formed, the piston rod being directly guided on the inner circumferential surface of the rod guide. The rod guide disclosed in Jikko Sho 55-23226 and Jikkai Hei 5-47569 is molded from a plate member, and comprises a supporting part which fits on the cylinder inner circumference and a cylindrical bearing part which supports the piston rod. To support the piston rod in the cylinder bearing part so that it is free to slide, the rod guide is further provided with a guide bush which fits on the inner circumference of the cylindrical bearing part.
However, the rod guide disclosed in Tokko Sho 62-38574 guides the piston rod directly, and therefore has inferior sliding properties.
Sliding improves if a guide bush is installed in the rod guide as disclosed in Jikko Sho 55-23226 and Jikkai Hei 5-47569. However, when the guide bush is engaged with the inner circumference of the cylindrical bearing part, the guide bush may fall out of the cylindrical bearing unit when the piston rod slides if there is only a weak fit. In order to prevent the guide bush from falling out, it is necessary to push the guide in as shown in Jikko Sho 55-23226 or to join the upper end of the cylindrical bearing part by caulking and provide a stopper at the lower end of the cylindrical bearing part as shown in Jikkai Hei 5-47569.
In order to prevent the guide bush falling out by caulking and stopper, the rod guide which is formed in one piece from a metal plate must have a caulking part and some means of attaching a stopper. This also involved complex assembly work and increased costs.
Moreover, the guide bush must be precision-machined so that the join surfaces fit exactly when the bush is pushed in. If it is not precision-machined, a gap may appear between the join surfaces or projecting ridges may be formed.
If such a gap is present between the join surfaces, oil in the cylinder may leak out and may considerably impair the function of the hydraulic damper. On the other hand, if there are projecting ridges, a gap may be formed between the guide bush and rod guide, and a similar problem may be caused. In this case, there is also a possibility that scratches may occur on the surface of the piston rod, smooth movement of the piston rod may be prevented, and the guide bush may deteriorate starting in the part where there are projections so as to cause early damage.
In addition, when the piston rod is supported free to slide in this way by the rod guide formed in one-piece from a plate and the guide bush which engages with it, as shown in Jikkai Hei 5-47569, the sliding resistance depends not only on the material of the guide bush but also on the manufacturing precision of both the rod guide and guide bush. Therefore, if both of these components are not precision machined, the piston rod cannot slide smoothly, noise is caused, and oil in the cylinder leaks.
It is therefore an object of this invention to support a piston rod such that it is free to slide smoothly even if a guide bush is not used.
It is also an object of this invention to prevent the risk of the guide bush falling out by making the guide bush unnecessary. This obviates the need for additional operations such as caulking, etc., improves productivity and decreases cost.
It is a further object of this invention to effectively prevent oil leakage, and to provide a piston rod with excellent durability which is able to slide smoothly for a long period of time without generating noise.